There exists a well known category of optical devices that perform optical filtering and can be tuned to select a narrow band of wavelengths from a wider wavelength spectrum. These devices are used in a variety of optical systems. Of specific interest are wavelength division multiplexed systems that operate typically over wavelength bands of tens of nanometers. These systems require optical performance monitoring (OPM) to ensure that signal power, signal wavelength, and signal to noise ratios (OSNR) are within specified limits. Other applications for tunable optical filters, inter alia, are for optical noise filtering, noise suppression, and wavelength division multiplexing.
For the purpose of describing this invention the focus will be OPM systems, more specifically, OPM systems for wavelength division multiplexed (WDM) systems. It will be understood that the invention is not so limited.
In WDM systems, basic system design assumes wavelength stability. However, a variety of dynamic changes occur due to temperature changes, component aging, electrical power variations, etc. For optimum system performance it is necessary to monitor these changes and adjust system parameters to account for them. To accomplish this, optical channel monitors (OCMs), also known as optical performance monitors (OPMs), may be used to measure critical information for the various channels in the WDM system. OPMs may monitor signal dynamics, determine system functionality, identify performance change, etc. In each case they typically provide feedback for controlling network elements to optimize operational performance. More specifically, these tunable optical filters scan the C-, L- and/or C+L-band wavelength range and precisely measure channel wavelength, power, and optical signal-to-noise ratio (OSNR).
Performance parameters for tunable optical filters are likewise important for the effectiveness of OPMs. These include adjacent channel isolation and non-adjacent channel isolation. Adjacent channel isolation is the difference between the minimum point in the pass channel and the maximum point in the adjacent channels over all relevant polarization states and over the temperature range of the specification. Non-adjacent channel isolation is the difference between the minimum point in the pass channel and the maximum point of non-adjacent channels. It is also useful for tunable optical filters used in these monitors to have very narrow bandwidth. That produces more information as the signal band is scanned by the tunable optical filter. On the other hand, for measuring optical power in a selected channel over a wider bandwidth, a tunable filter with a correspondingly wider bandwidth makes that measurement simpler. This is among several trade-offs encountered in OPM design. There is also the ubiquitous trade-off of cost.
In a multichannel system, the monitor for each channel has several operating elements. Recognizing that each of these elements are multiplied many times over in assessing the overall system cost, an apparently small cost efficiency in the design of the monitor is likewise multiplied to reach the overall cost impact. In some cases increasing the complexity of the monitors by adding elements may result in a system cost reduction depending on the relative costs of the elements.